They said they're investigating whether the executives delayed releasing information about VW's manipulation of software to cheat on emissions tests, and its possible financial implications on the holding company. German securities law requires companies to broadly and quickly disclose information that could affect decisions to buy or sell the company's shares.

Porsche SE said in a statement that "we are convinced that we have duly fulfilled our capital market disclosure requirements."

In addition to Mueller, who is strategy and development chief at the holding company, those under investigation are Hans Dieter Poetsch, who is CEO of the holding company as well as Volkswagen board chairman, and Martin Winterkorn, the former Volkswagen and holding company chief executive who quit after the scandal broke in 2015.

The Porsche holding company is distinct from Porsche sports car brand, which is now part of Volkswagen itself. The holding company's shareholders are members of the Piech and Porsche families, descendants of automotive pioneer Ferdinand Porsche.

Volkswagen has admitted equipping around 11 million cars worldwide with software that sensed when cars were on test stands and turned emission controls up, then turned the controls off during every day driving to improve performance.

It has agreed to at least $16 billion in civil settlements with environmental authorities and car owners in the United States, and to a $4.3 billion criminal penalty. Seven Volkswagen executives have been criminally charged in the U.S. The company also faces investor lawsuits in Germany alleging it did not inform shareholders of the scandal quickly enough. Volkswagen says it met its duties.

The company apologized for the scandal and says it is changing its culture and practices.